Light emitting device with molded wavelength converting layer

ABSTRACT

A flexible film comprising a wavelength converting material is positioned over a light source. The flexible film is conformed to a predetermined shape. In some embodiments, the light source is a light emitting diode mounted on a support substrate. The diode is aligned with an indentation in a mold such that the flexible film is disposed between the support substrate and the mold. Transparent molding material is disposed between the support substrate and the mold. The support substrate and the mold are pressed together to cause the molding material to fill the indentation. The flexible film conforms to the shape of the light source or the mold.

BACKGROUND

1. Field of Invention

The present invention relates to a wavelength converted light emittingdevice.

2. Description of Related Art

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

FIG. 1 illustrates an LED described in more detail in U.S. Pat. No.7,352,011, which is incorporated herein by reference. An LED 10 ismounted on a support structure (not shown in FIG. 1). An inner lens 64is molded over LED 10. Lens 64 is formed as follows: a mold in the shapeof lens 64 is positioned over LED 10. The mold may be lined with anon-stick film. The mold is filled with a suitable transparentheat-curable liquid lens material such as silicone or epoxy. A vacuumseal is created between the periphery of the support structure and themold, and the two pieces are pressed against each other so that each LEDdie 10 is inserted into the liquid lens material and the lens materialis under compression. The mold is then heated to about 150 degreescentigrade (or other suitable temperature) for a time to harden the lensmaterial. The support structure is then separated from the mold. In aseparate molding step (using a mold with deeper and wider indentations)an outer phosphor/silicone shell 66 of any thickness is formed directlyover the inner lens 64. An outer lens 68 may be formed over thephosphor/silicone shell 66 using another mold to further shape the beam.

SUMMARY

It is an object of the invention to provide a wavelength convertingmaterial in a flexible film which is molded over a light source. In someembodiments, the thickness of the wavelength converting material may betightly controlled.

In embodiments of the invention, a flexible film comprising a wavelengthconverting material is positioned over a light source. The flexible filmis conformed to a predetermined shape. In some embodiments, the lightsource is a light emitting diode mounted on a support substrate. Thediode is aligned with an indentation in a mold such that the flexiblefilm is disposed between the support substrate and the mold. Transparentmolding material is disposed between the support substrate and the mold.The support substrate and the mold are pressed together to cause themolding material to fill the indentation. The flexible film conforms tothe shape of the light source or the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art LED mounted on a support structure andcovered with a molded lens.

FIG. 2 illustrates a flexible wavelength converting film disposedbetween a mold and multiple LEDs mounted on a support substrate.

FIG. 3 illustrates molding material disposed between LEDs mounted on asupport substrate and a wavelength converting film lining a mold.

FIG. 4 illustrates the molding material of FIG. 3 molded into lensesover the LEDs.

FIG. 5 illustrates the structure of FIG. 4 after removing the mold.

FIG. 6 illustrates the structure of FIG. 5 after removing a supportfilm.

FIG. 7 illustrates a flexible wavelength converting film positioned overLEDs mounted on a support substrate.

FIG. 8 illustrates the structure of FIG. 7 flipped over and positionedover a mold.

FIG. 9 illustrates molding material disposed between a wavelengthconverting film and a mold.

FIG. 10 illustrates the molding material of FIG. 9 molded into lensesover the LEDs.

FIG. 11 illustrates the structure of FIG. 10 after removing the mold.

FIG. 12 illustrates a first wavelength converting film positioned underLEDs mounted on a support substrate and a second wavelength convertingfilm positioned over a mold.

FIG. 13 illustrates molding material disposed between the two wavelengthconverting films illustrated in FIG. 13.

FIG. 14 illustrates the molding material of FIG. 13 formed into lensesover the LEDs.

FIG. 15 illustrates the structure of FIG. 14 after removing the mold.

FIG. 16 illustrates the structure of FIG. 15 after removing a supportfilm.

DETAILED DESCRIPTION

In the device illustrated in FIG. 1, three molds are used, a first moldto form transparent layer 64, a second, larger mold to form phosphorlayer 66, and a third still larger mold to form transparent layer 68.The thickness of phosphor layer 66 determines the characteristics suchas the color temperature of the light emitted by the device. Thethickness of phosphor layer 66 can vary based on variations in thealignment of the phosphor mold, which may undesirably alter thecharacteristics of the emitted light. For example, phosphor layer 66 maybe in the range of 100 μm thick. The mold-alignment tolerance of thephosphor mold may be in the range of 30-50 μm. The high mold-alignmenttolerance relative to the target thickness of the phosphor can causevariations in the thickness of the phosphor that undesirably shift thecolor point of light emitted from the device.

In embodiments of the invention, a wavelength converting material isformed into a flexible film which is molded over a light source such asa semiconductor light emitting device. The film may be fully orpartially cured transparent material, such as silicone, which is filledwith phosphor. The wavelength converting film may be formed separatefrom the molding process, which permits tight control of the thicknessof the wavelength converting film as well as testing and verification ofthe characteristics of the film prior to molding. Molding and otherprocessing steps according to the invention may be performed asdescribed in U.S. Pat. No. 7,352,011. In some embodiments, thewavelength converting flexible film is formed on a support film such asan ethylene tetrafluoroethylene film, for example by spreading a mixtureof phosphor and silicone over the support film then fully or partiallycuring the silicone in the wavelength converting film. The wavelengthconverting material may be temporarily protected by another ethylenetetrafluorethylene film, which is removed before the processingdescribed below. After processing, the support film is generally peeledaway from the wavelength converting film. In some embodiments, thesupport films serves as a release film, which releases a molded lensfrom the mold.

In some embodiments, a heat dissipating material such as diamond may beadded to the wavelength converting film. In some embodiments, a materialsuch as diamond may be added to the wavelength converting film to adjustthe refractive index of the film, to improve optical transmission,and/or to improve conversion.

As a preliminary matter, a light source is provided. Though in theexamples below the light source is an LED, any suitable light source maybe used. Also, though in the examples below the light source is aIII-nitride LED that emits blue or UV light, semiconductor lightemitting devices besides LEDs and semiconductor light emitting devicesmade from other materials systems such as other III-V materials,III-phosphide, III-arsenide, II-VI materials, or Si-based materials maybe used.

A conventional III-nitride LED 10 is formed by first growing asemiconductor structure on a growth substrate. An n-type region istypically grown first and may include multiple layers of differentcompositions and dopant concentration including, for example,preparation layers such as buffer layers or nucleation layers, which maybe n-type or not intentionally doped, release layers designed tofacilitate later release of the substrate or thinning of thesemiconductor structure after substrate removal, and n- or even p-typedevice layers designed for particular optical or electrical propertiesdesirable for the light emitting region to efficiently emit light. Alight emitting or active region is grown over the n-type region.Examples of suitable light emitting regions include a single thick orthin light emitting layer, or a multiple quantum well light emittingregion including multiple thin or thick quantum well light emittinglayers separated by barrier layers. A p-type region is grown over thelight emitting region. Like the n-type region, the p-type region mayinclude multiple layers of different composition, thickness, and dopantconcentration, including layers that are not intentionally doped, orn-type layers.

Various techniques are used to gain electrical access to the n-layers.Portions of the p-layers and active layer are etched away to expose ann-layer for metallization. In this way the p-contact and n-contact areon the same side of the chip and can be directly electrically attachedto contact pads on a suitable mount. The device can be mounted such thatlight is extracted from the top surface (i.e. the surface on which theelectrical contacts are formed) or the bottom surface (i.e. a flip chipwith reflective contacts). The growth substrate may be removed from thesemiconductor structure or may remain a part of the completed device.After substrate removal, the semiconductor structure may be thinned, andthe surface of the n-type region exposed by removing the substrate maybe textured to improve light extraction, for example by roughening orforming a photonic crystal structure. In contrast, in a verticalinjection LED, an n-contact is formed on one side of the semiconductorstructure, and a p-contact is formed on the other side of thesemiconductor structure. Electrical contact to one of the p- orn-contacts is typically made with a wire or a metal bridge, and theother contact is directly bonded to a contact pad on the mount. Thoughthe examples below include a flip chip device from which the growthsubstrate has been removed, any suitable device structure may be used.

One or more wavelength converting materials such as phosphors or dyesmay be included in the flexible film molded over the LED.Non-wavelength-converting materials such as TiO_(x) particles may beincluded in the flexible film, for example to cause light scattering orto improve the off-state white appearance of the device. The flexiblefilm may be textured to change the optical properties, for example byembossing microlenses to improve light extraction.

An LED combined with one or more wavelength converting materials may beused to create white light or monochromatic light of other colors. Allor only a portion of the light emitted by the LED may be converted bythe wavelength converting material. Though in the examples below thewavelength converting materials are powder phosphors, any suitablewavelength converting material may be used. Unconverted light may bepart of the final spectrum of light, though it need not be. Examples ofcommon combinations include a blue-emitting LED combined with ayellow-emitting phosphor, a blue-emitting LED combined with green- andred-emitting phosphors, a UV-emitting LED combined with blue- andyellow-emitting phosphors, and a UV-emitting LED combined with blue-,green-, and red-emitting phosphors. Wavelength converting materialsemitting other colors of light may be added to tailor the spectrum oflight emitted from the device. Multiple wavelength converting materialsmay be mixed and formed in a single film, formed as discrete layers in asingle film, or formed as mixed or discrete layers in multiple films.

Wavelength converting materials formed in a flexible film may becombined in a device with other conventional wavelength convertingmaterials, such as pre-formed ceramic phosphor layers that are glued orbonded to the LED, or a powder phosphor disposed in an organicencapsulant that is stenciled, screen printed, sprayed, sedimented,evaporated, sputtered, or otherwise dispensed over the LED.

FIGS. 2-6 illustrate forming a first embodiment of the invention.

In FIG. 2, several LEDs 10 mounted on a support substrate 12 are alignedwith indentations 22 corresponding to each LED 10 in a mold 20. Awavelength converting flexible film 14 is disposed on a support film 16.The wavelength converting film 14 and support film 16 are disposedbetween support substrate 12 and mold 20. In some embodiments, thewavelength converting film is fully cured. In some embodiments, supportfilm 16 is a material which prevents the molding material 24 describedin FIG. 3 from adhering to mold 20, such as ethylenetetrafluoroethylene.

The thickness of wavelength converting film 14 depends on the wavelengthconverting material used and the desired characteristics of the lightemitted from the device. The thickness of wavelength converting film 14may be between 10 and 200 μm in some embodiments, between 40 and 60 μmin some embodiments, and 50 μm in some embodiments.

In FIG. 3, support film 16 and wavelength converting film 14 are pressedagainst mold 20 such that they line the indentations 22 in mold 20. Forexample, support film 16 and wavelength converting film 14 may bestretched over the mold indentations, then a reduced pressureenvironment or vacuum may be created which pulls the films into the moldindentations as illustrated in FIG. 3. Molding material 24 is disposedbetween mold 20 and support substrate 12. Though indentations 22 maytake any suitable shape, typically indentations 22 are shaped as lensesand molding material 24 is transparent. Though molding material 24 isoften silicone, any suitable molding material may be used.

In FIG. 4 mold 20 and support substrate 12 are pressed together to forcemolding material 24 to fill indentations 22 in mold 20 and to adhere toLEDs 10 and support substrate 12. A vacuum or reduced pressureenvironment may be applied to draw molding material 24 into indentations22. Lenses 26 are formed on each LED 10. Molding material 24 may becured as appropriate to the molding material, for example by heating, toform solid lenses 26.

In FIG. 5, mold 20 is removed, leaving lenses 26 formed of moldingmaterial 24 over each LED 10. Wavelength converting film 14 and supportfilm 16 are disposed over each lens 26 and between adjacent lenses.

In FIG. 6, support film 16 is removed.

In the device illustrated in FIG. 6, wavelength converting film 14 isspaced apart from LED 10 by lens 26. FIGS. 7-11 illustrate forming adevice with the wavelength converting film in direct contact with theLED.

In FIG. 7 a flexible wavelength converting film 28 is positioned overLEDs 10, which are attached to support substrate 12. Wavelengthconverting film 28 may have the same characteristics as the wavelengthconverting film 14 described above, and may be formed on a support film.In some embodiments, wavelength converting film 28 is only partiallycured.

In FIG. 8 LEDs 10 are aligned with indentations 22 corresponding to eachLED 10 in a mold 20.

In FIG. 9 molding material 24 is disposed between mold 20 and wavelengthconverting film 28. Mold 20 may be lined with an optional release film,not shown in FIG. 9.

In FIG. 10, mold 20 and support substrate 12 are pressed together toforce molding material 24 to take the shape of indentations 22 in mold20 to form lenses 26 over each LED 10. Molding material 24 forceswavelength converting film 28 to conform to the shape of LEDs 10 andsupport substrate 12 and to adhere to LEDs 10 and support substrate 12.Molding material 24 adheres to wavelength converting film 28. Moldingmaterial 24 may be cured as described above.

In FIG. 11, mold 20 is removed. A release film remaining over lenses 26is also removed. In the devices illustrated in FIG. 11, wavelengthconverting film 28 is in direct contact with LEDs 10 and supportsubstrate 12 between LEDs 10. Lenses 26 are disposed over wavelengthconverting film 28.

FIGS. 12-16 illustrate forming a device with two wavelength convertingfilms.

In FIG. 12, a flexible wavelength converting film 28 is positioned overLEDs 10 and support substrate 12, as described above in reference toFIG. 7. Wavelength converting film 28 may be formed on a support filmand in some embodiments is partially cured. A wavelength converting film14 and a support film 16 as described above in reference to FIG. 2 arepositioned over a mold 20. In some embodiments, wavelength convertingfilm 14 is fully cured. Indentations 22 in mold 20 are aligned with LEDs10.

In FIG. 13 wavelength converting film 14 and support film 16 are pressedinto the indentations 22 in mold 20 such that they line mold 20, asdescribed above in reference to FIG. 3. Molding material 24 is dispensedbetween wavelength converting film 28 and wavelength converting film 14.

In FIG. 14 mold 20 and support structure 12 are pressed together.Molding material 24 fills the indentations 22 in mold 20 to form lensesover LEDs 10. Molding material 24 forces wavelength converting film 28to conform to the shape of LEDs 10 and support substrate 12.

In FIG. 15, mold 20 is removed.

In FIG. 16, support film 16 is removed. In the device illustrated inFIG. 16, a first wavelength converting film 28 directly contacts the topsurface of LEDs 10. Lenses 26 are disposed over first wavelengthconverting film 28. A second wavelength converting film 14 is disposedover lenses 26. The first and second wavelength converting films mayinclude different wavelength converting materials. In one example, LEDs10 may emit blue light, one of the wavelength converting films mayinclude a yellow- or green-emitting phosphor, and the other of thewavelength converting films may include a red-emitting phosphor.

In some embodiments, an optional, additional transparent layer may bemolded over the devices illustrated in FIGS. 6 and 16 to protectwavelength converting film 14. The additional transparent layer may beshaped to increase extraction efficiency and/or sized to provide adesired source size. The material may be selected to improve color vs.angle of the light emitted from the device or to increase overall lightoutput. All or part of the surface of the additional transparent layermay be textured to improve light extraction. In some embodiments, lens26 has smaller index of refraction than the additional transparentlayer, which may reduce the number of unconverted blue photons scatteredback toward the LED where they can be lost to absorption. In oneexample, the index of refraction of lens 26 is 1.4 or lower and theindex of refraction of the additional transparent layer is 1.5 orhigher.

A molded wavelength converting film as described in the embodimentsabove may have several advantages over conventional wavelengthconverting layers. The flexible wavelength converting films may beformed separate from the molding process. Color measurements can beperformed in advance to achieve a desired color and films may betailored to a given LED emission color. Wavelength converting films maybe manufactured in large areas at low cost. The distance between LED 10and wavelength converting film 14, the thickness of the wavelengthconverting films, and the uniformity of the thickness of the wavelengthconverting films over the LED may be tightly controlled and optimized toimprove extraction efficiency, color control, and source size.

Spacing the wavelength converting film apart from the light source in anear-remote arrangement as illustrated in FIG. 6 as opposed to disposingthe wavelength converting film directly on the LED may also haveadvantages. For example, a near-remote wavelength converting film may beexposed to less heat from the LED, which may allow the use of wavelengthconverting materials that are temperature sensitive. The photon densityin a near-remote wavelength converting layer may be decreased, which mayincrease light output from the device. The absorption cross section ofthe LED may be reduced with a near-remote wavelength converting film,which may increase extraction efficiency. In devices with a materialsuch as TiO_(x) included in the wavelength converting film to improvethe off-state white appearance of the device, light loss for a givenwhiteness may be reduced due to the reduced absorption cross section.Color vs. angle dependence of the emitted light in a near-remotewavelength converted device may be reduced.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is being claimed is:
 1. A method comprising: positioning a flexiblefilm over a light source, the flexible film comprising a transparentmaterial filled with wavelength converting material; and aligning thelight source with an indentation in a mold such that the flexible filmis disposed between the light source and the mold; disposing transparentmolding material between the light source and the mold; and pressing thelight source and the mold together to cause the molding material to fillthe indentation and to cause the flexible film to conform to apredetermined shape.
 2. The method of claim 1 wherein the light sourcecomprises a III-nitride light emitting diode mounted on a supportsubstrate.
 3. The method of claim 2 wherein the flexible film is indirect contact with the light emitting diode and conforms to a shape ofthe light emitting diode.
 4. The method of claim 2 wherein the flexiblefilm is in direct contact with the mold and conforms to a shape of theindentation.
 5. The method of claim 2 wherein the flexible filmcomprises a powder phosphor disposed in a silicone film.
 6. The methodof claim 5 wherein the silicone film is fully cured prior to saidpositioning.
 7. The method of claim 5 wherein the silicone film ispartially cured prior to said positioning.
 8. The method of claim 5wherein the flexible film further comprises a non-wavelength convertingmaterial, wherein the non-wavelength converting material changes anindex of refraction of the flexible film.
 9. The method of claim 2wherein the flexible film is disposed on a support film.
 10. The methodof claim 9 further comprising removing the support film after pressingthe light source and the mold together.
 11. The method of claim 2wherein the indentation is shaped as a lens.
 12. The method of claim 2wherein the flexible film is a first flexible film in direct contactwith the light emitting diode, the method further comprising:positioning a second flexible film comprising a wavelength convertingmaterial in direct contact with the mold; wherein the transparentmolding material is disposed between the first flexible film and thesecond flexible film.
 13. The method of claim 2 wherein pressing thelight source and the mold together to cause the molding material to fillthe indentation and to cause the flexible film to conform to apredetermined shape comprises creating a reduced pressure environmentwhich draws the molding material into the indentation.
 14. The method ofclaim 1 wherein the flexible film is made by evenly spreading a mixtureof phosphor and silicone over a support film then at least partiallycuring the silicone.
 15. The method of claim 1 further comprisingheating the mold after pressing the light source and the mold together.16. The method of claim 15 wherein the mold is heated to a temperaturethat hardens the molding material.
 17. The method of claim 1 wherein:the light source is a first light source; the method further comprisesmounting the first light source and a second light source on a supportsubstrate; aligning the light source with an indentation in a moldcomprises aligning the mold such that the first light source is alignedwith a first indentation and the second light source is aligned with asecond indentation; and positioning a flexible film over a light sourcecomprises positioning the flexible film such that wavelength convertingmaterial is positioned in a space between the first light source and thesecond light source.
 18. The method of claim 17 wherein after pressingthe light source and the mold together, a continuous layer of wavelengthconverting material is disposed over the first light source, the secondlight source, and the space between the first light source and thesecond light source.
 19. The method of claim 1 wherein a thickness ofwavelength converting material in the flexible film is uniform.